1. Field of the Invention
The present invention relates to a hydraulic accumulator used in hydraulic systems for automobiles and other industrial vehicles and equipment, and a flexible partition member in the form of a diaphragm or a bag or bladder used in such a hydraulic accumulator.
2. Discussion of the Prior Art
A hydraulic accumulator has a function of storing a hydraulic fluid under pressure, and generally includes a metallic shell, and a flexible partition member such as a rubber diaphragm or a rubber bag or bladder, which divides the interior of the shell into a gas chamber and a liquid chamber. The diaphragm or bladder (hereinafter generically referred to as "flexible partition member") is formed of a suitable rubber material into a desired shape having a relatively small thickness. The flexible partition member is fixed within the shell, and cooperates with the wall of the shell to define the gas and liquid chambers. The gas chamber is charged with a suitable gas such as N.sub.2 gas, while a pressurized oil or liquid is stored in the liquid chamber. The oil is fed into the liquid chamber from a hydraulic pressure source, and the compressed gas acts as a cushion or a spring. During use of the accumulator, the gas trapped in the gas chamber may gradually permeate through the thin flexible partition member, and become incapable of performing its function, due to lowering of the gas pressure which results from the permeation of the gas through the partition member.
One solution to the above drawback is to add a suitable additive or additives to the rubber composition of the flexible partition member, in an attempt to improve the resistance of the partition member to the gas permeation. Although the additives may improve the gas permeation resistance (gas imperviousness or impermeability) of the partition member, the flexibility of the partition member tends to be lowered, and the partition member is likely to crack after a relatively long period of use within the accumulator shell. This is also a problem.
To solve the above problem, it is proposed to interpose an ethylene-vinyl alcohol copolymer resin layer between adjacent thin rubber layers of a flexible partition member, such that the resin layer is bonded to the rubber layers by adhesive layers. An example of a multiple-layered rubber-resin structure is disclosed in JP-A-2-165948. Where this rubber-resin structure is used for a flexible diaphragm or bladder for a hydraulic accumulator, the ethylene-vinyl alcohol resin layer or layers which have a high degree of gas impermeability enable the gas charge in the accumulator shell to maintain its pressure above the lower limit, for a prolonged period of use of the accumulator. However, the ethylene-vinyl alcohol copolymer resin layers suffer from cracking and separation from the rubber layers, after a long use during which the diaphragm or bladder experiences bending deformation or displacement a large number of times. Accordingly, the life expectancy of the accumulator using such diaphragm or bladder is shortened.
An impervious resilient membrane is disclosed in U.S. Pat. No. 5,036,110. This membrane includes two first layers each consisting of a first material, and a second layer interposed between the first layers and consisting of a second material. The first material is selected from the group consisting of thermoplastic polyurethanes, block amide polyethers, flexible polyesters and mixture thereof, while the second material is selected from the group consisting of copolymers of ethylene and vinyl alcohol, polyamides, polyvinylidene chloride and mixture thereof.
Although the second layer is impervious to a gas, some of the first materials indicated above have an insufficient heat resistance and tend to be deteriorated during use of an accumulator in which the temperature of a stored pressurized fluid is relatively high. Further, the other first materials may be softened due to exposure to a hydraulic fluid such as a brake fluid, and tend to have a relatively short service life.